C++ GlobalVariable::getSection方法代码示例

bool ConstantMerge::runOnModule(Module &M) {
  // Find all the globals that are marked "used".  These cannot be merged.
  SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
  FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
  FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
  
  // Map unique constant/section pairs to globals.  We don't want to merge
  // globals in different sections.
  DenseMap<Constant*, GlobalVariable*> CMap;

  // Replacements - This vector contains a list of replacements to perform.
  SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;

  bool MadeChange = false;

  // Iterate constant merging while we are still making progress.  Merging two
  // constants together may allow us to merge other constants together if the
  // second level constants have initializers which point to the globals that
  // were just merged.
  while (1) {
    // First pass: identify all globals that can be merged together, filling in
    // the Replacements vector.  We cannot do the replacement in this pass
    // because doing so may cause initializers of other globals to be rewritten,
    // invalidating the Constant* pointers in CMap.
    //
    for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
         GVI != E; ) {
      GlobalVariable *GV = GVI++;
      
      // If this GV is dead, remove it.
      GV->removeDeadConstantUsers();
      if (GV->use_empty() && GV->hasLocalLinkage()) {
        GV->eraseFromParent();
        continue;
      }
      
      // Only process constants with initializers in the default addres space.
      if (!GV->isConstant() ||!GV->hasDefinitiveInitializer() ||
          GV->getType()->getAddressSpace() != 0 || !GV->getSection().empty() ||
          // Don't touch values marked with attribute(used).
          UsedGlobals.count(GV))
        continue;
      
      
      
      Constant *Init = GV->getInitializer();

      // Check to see if the initializer is already known.
      GlobalVariable *&Slot = CMap[Init];

      if (Slot == 0) {    // Nope, add it to the map.
        Slot = GV;
      } else if (GV->hasLocalLinkage()) {    // Yup, this is a duplicate!
        // Make all uses of the duplicate constant use the canonical version.
        Replacements.push_back(std::make_pair(GV, Slot));
      }
    }

    if (Replacements.empty())
      return MadeChange;
    CMap.clear();

    // Now that we have figured out which replacements must be made, do them all
    // now.  This avoid invalidating the pointers in CMap, which are unneeded
    // now.
    for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
      // Eliminate any uses of the dead global.
      Replacements[i].first->replaceAllUsesWith(Replacements[i].second);

      // Delete the global value from the module.
      Replacements[i].first->eraseFromParent();
    }

    NumMerged += Replacements.size();
    Replacements.clear();
  }
}